1. Field of the Invention
The present invention relates to an ion sensor and, more particularly, to an ion sensor of the type wherein an ion concentration in a solution is measured by an electrode potential response.
2. Description of the Prior Art
Conventional ion sensors of the type wherein an ion concentration in a solution is measured by electrode potential response include various ion sensors such as a hydrogen ion sensor, a potassium ion sensor, a calcium ion sensor, a sodium ion sensor, or a chloride ion sensor.
As a hydrogen ion sensor, glass electrodes are widely used, wherein a standard solution having a stable pH and a stable chloride activity is held with an internal reference electrode (Ag/AgCl electrode) in a standard liquid chamber formed by a glass membrane. The sensor is dipped in a sample solution, and the pH value in the sample solution can be measured by the potential difference of both reference electrode between in the internal standard and the outer sample solutions. However, when glass electrodes are used, the glass membrane can be easily damaged or contaminated and their use in alkaline solutions is limited. When a highly viscous solution containing an adsorbing substance (e.g., blood) is to be measured, the measurement precision is degraded within a short period of time. In order to prevent this degradation, substances attached to the glass membrane must be removed every 30 minutes during measurement. In addition, since the glass type electrode has a standard internal solution chamber, it cannot be reduced in size beyond a certain limit.
A pH sensor of the type using a hydrogen ion carrier film in place of a glass membrane is disclosed in, e.g., U.S. Pat. No. 3,743,558, Japanese Patent Disclosure No. 47-7549, and J. Appl. Physiology 40, 14. The hydrogen ion carrier film is obtained by adding a hydrogen ion carrier substance in a polymer film and hydrogen ions from the inner solution permeate the film to reach a reference electrode so as to measure the hydrogen ion concentration. However, in this pH sensor, the carrier film components tend to elute, presenting a problem of poor measurement reproducibility.
As an improvement over this type of pH sensor, a liquid film electrode is reported in Analytica Chimica Acta, 131, 111-116 (1981). A potassium ion sensor using a liquid film electrode is also reported in Analytical Chemistry 46, 2223-2224 (1974). However, both types of sensor have a standard internal solution chamber and cannot be rendered compact in size.